Certain photolithography parameters, e.g., the variation of critical dimension (“CD”) printed with pitch, otherwise sometimes referred to as Optical Proximity Effect (OPE), e.g., in a scanner imaging system, shows a behavior that is characteristic of the imaging and process conditions and is sensitive to variations in those conditions. Maintaining stable process conditions can improve the effectiveness of mask Optical Proximity Correction (OPC). One of the factors which affects the OPE is the spectral bandwidth of the light source. To date, passive bandwidth stabilization techniques have been effective in meeting OPE control requirements. However, future tighter OPE specifications will require advanced bandwidth control techniques. According to aspects of an embodiment of the disclosed subject matter applicants propose developments in active stabilization of bandwidth, e.g., in single chambered laser systems, such as applicants' assignee's Cymer 7XXX laser systems, e.g., the 7010 and/or in dual chamber laser systems, e.g., master oscillator and amplifier gain medium laser systems such as master oscillator-power amplifier (“MOPA”) such as Cymer XLA 1XX, 2XX or 3XX laser systems or master oscillator power oscillator (“MOPO”) laser systems.
The recent work of Huggins et al., “Effects of laser bandwidth on OPE in a modern lithography tool.”, Optical Microlithography XVIII (2006), describes how controlling the spectral properties of the laser light, specifically E95 bandwidth, has an effect of similar magnitude to those from other control parameters, such as focus shift, dose shift and partial coherence shift. The bandwidth metric, E95, is defined as the width of the spectrum (typically in picometers) that contains 95% of the integrated spectral intensity. A second bandwidth metric that is commonly employed is the Full Width at Half-Maximum (FWHM), which, although easier to measure than E95, does not affect OPE as significantly.
To date, as noted, passive bandwidth stabilization techniques have been effective in meeting OPE control requirements. However, applicants believe that future tighter OPE specifications will require active control techniques to not only improve the stability of E95 bandwidth, but also regulate E95 bandwidth to a desired setpoint (i.e., within a selected very narrow range. FIG. 6 by way of example relates the concepts of stability and setpoint regulation to those of passive and active control. The left most plot (Nominal) depicts the E95 variability versus time as a system baseline. The middle plot (Passive) illustrates that with passive improvements one may, e.g., aim to improve E95 stability, reduce the E95 variability, and usually lower the E95 setpoint. The right most plot (Active) illustrates by way of example that the aim of active control methods is to further refine the E95 stability and variability, and allow dynamic selection of the E95 set point.
Lambda Physik AG patent U.S. Pat. No. 6,490,308 discusses various means for extending gas life.